1. Field:
This invention relates to NMR imaging techniques and is particularly directed to such a technique adapted to display an image revealing the presence of tissues, such as fat, or aerated lung tissue, characterized by internal magnetic inhomogeneities.
2. State of the Art:
Nuclear magnetic resonance (NMR) has been recognized as a useful phenomenon in various physical and chemical fields. A variety of NMR-based analytical techniques have evolved in disparate physical and chemical disciplines, including medical-related sciences, notably biophysics and biochemistry. Among these techniques is the measurement of water content (or mobile proton content) in various organic substances, including tissues. Available NMR apparatus is capable of producing complicated pulse sequences, automatically recording the NMR signals resulting from applying those pulse sequences to a target and then processing those signals "on line" to produce an image.
The state of the art of NMR imaging in biomedicine is reflected in the book "NMR Imaging in Biomedicine", P. Mansfield and P. G. Morris, 1982 Academic Press, New York, N.Y. (Library of Congress Catolog Card No. 65-26774). The disclosure of that text book is incorporated herein by reference.
The method of NMR imaging which forms the background for this invention involves placing a specimen containing a selected nuclear species within an approximately homogeneous magnetic field "H.sub.O " to effect a net nuclear magnetization of that nuclear species. The nuclear species selected for biomedical NMR applications is ordinarily water, but other species such as phosphorous, fluorine and sodium may be selected. For other NMR applications, a nuclear species may be selected from any of a hundred or more substances which are known to have nonintegral spins. The nuclei of the selected nuclear species are irradiated within a target region of the specimen during a "90.degree. pulse" with a radio frequency varying magnetic field to orient the net nuclear magnetization of the selected species within the target region to a direction approximately normal to the homogeneous magnetic field.
The term "90.degree. pulse" is used herein in the sense it is conventionally used in NMR technology to indicate the intensity and duration of a pulse of energy applied to the nuclei to effect a maximum NMR decay signal [usually called a free induction decay (FID) signal]. The duration for which the 90.degree. pulse is applied is referred to herein as a "90.degree. pulse duration." Following the application of the 90.degree. pulse, a first magnetic gradient is applied to the homogeneous field to effect a first spatially varying resultant field. The net nuclear magnetization is permitted to precess about this first resultant field during a first free induction decay (FID) interval, thereby to induce a first NMR signal voltage across a receiver device. The receiver device is usually a tuned pickup, such as a tuned rf receiving coil, but it may be embodied as some other pickup or antenna device. The first magnetic gradient is terminated after the first NMR signal has decayed. The nuclei are then irradiated within the target region by a 180.degree. pulse during a "180.degree. pulse duration" with a radio frequency varying magnetic field to cause them to rephase during a second FID interval. The term "180.degree. pulse" is used herein in the sense it is conventionally used in NMR terminology to mean the intensity and duration of a pulse of energy applied to the nuclei to effect a rotation of all spins by 180.degree. about an axis perpendicular to the homogeneous magnetic field (H.sub.O). The duration that this pulse is applied is called a "180.degree. pulse duration." A second magnetic gradient is applied to the homogeneous magnetic field during the second FID interval to effect a second spatially varying resultant field. During this time, a second NMR signal voltage is ordinarily present across the receiver device. The first and second NMR signals are ordinarily ignored, but a resultant NMR echo signal, which occurs subsequently, is detected and processed in various ways.
Ordinarily, a large number of echo signals are induced from various target regions within the specimen, and the signals are resolved into NMR images of varying types. Typically, a sequence of individual procedures ("experiments") is performed. Each experiment generates an NMR echo signal. Field gradients are imposed upon the homogeneous magnetic field, and those field gradients are systematically adjusted to a plurality of net field conditions, each of which is maintained for a discrete interval of time corresponding to an individual experiment. The foregoing sequence of steps is repeated during each of the net field conditions in accordance with an NMR imaging strategy to produce a corresponding plurality of echo NMR signals, each of which emanates from a discrete target region within the specimen. The various imaging strategies available may result in echo signals which can be resolved into zero, one, two, three or multidimensional images. The electronic and logic circuitry utilized in connection with modern CAT scanning techniques are commonly utilized to process NMR echo signal data. In this fashion, NMR signal data are resolved to produce NMR images analogous to conventional CAT scanning images. Other general or specialized logic circuitry may be used in place of or to supplement conventional CAT scanning logic circuitry. There are currently available commercial NMR scanners which include logic circuits specifically adapted to NMR imaging.
It is known to apply NMR imaging techniques to the study of various systems, including biological and chemical systems. The basic approach to producing images in these systems is generally similar.